Semiconductor nanoparticles, such as CdSe crystals with diameters in the range of 1–7 nm, are important new materials that have a wide variety of applications, particularly in the biological arena. Of the many unique properties of these materials, the photophysical characteristics are some of the most useful. Specifically, these materials can display intense luminescent emission that is particle size-dependent and particle composition-dependent, can have an extremely narrow bandwidth, and can be environmentally insensitive; such emissions can be efficiently excited with electromagnetic radiation having a shorter wavelength than the highest energy emitter in the material. These properties allow for the use of semiconductor nanocrystals as ultra-sensitive luminescent reporters of biological states and processes in highly multiplexed systems.
Some bare nanocrystals, i.e., nanocrystal cores, do not display sufficiently intense or stable emission, however, for these applications. In fact, the environments required for many applications can actually lead to the complete destruction of these materials. A key innovation that increases the usefulness of the nanocrystals is the addition of an inorganic shell over the core. The shell is composed of a material appropriately chosen to be preferably electronically insulating (through augmented redox properties, for example), optically non-interfering, chemically stable, and lattice-matched to the underlying material. This last property is important, since epitaxial growth of the shell is often desirable. Furthermore, matching the lattices, i.e., minimizing the differences between the shell and core crystallographic lattices, minimizes the likelihood of local defects, the shell cracking or forming long-range defects.
Considerable resources have been devoted to optimizing nanoparticle core synthesis. Much of the effort has been focused on optimization of key physiochemical properties in the resultant materials. For example, intense, narrow emission bands resulting from photo-excitation are commonly desirable. Physical factors impacting the emission characteristics include the crystallinity of the material, core-shell interface defects, surface imperfections or “traps” that enhance nonradiative deactivation pathways (or inefficient radiative pathways), the gross morphologies of the particles, and the presence of impurities. The use of an inorganic shell has been an extremely important innovation in this area, as its use has resulted in dramatic improvements in the aforementioned properties and provides improved environmental insensitivity, chemical and photochemical stability, reduced self-quenching characteristics, and the like.
Shell overcoating methodologies have, to date, been relatively rudimentary. Shell composition, thickness, and quality (e.g., crystallinity, particle coverage) have been poorly controlled, and the mechanism(s) of their effects on particle luminescence poorly understood. The impact of overcoating on underlying luminescence energies has been controlled only sparsely through choice and degree of overcoating materials based on a small set of criteria.
Hines et al. (1996) “Synthesis and Characterization of Strongly Luminescing ZnS-Capped CdSe Nanocrystals,” J. Phys. Chem. 100:468 describe the preparation of a ZnS-capped CdSe nanocrystal that exhibits a significant improvement in luminescence yields: up to 50% quantum yield at room temperature. Unfortunately, the quality of the emitted light remains unacceptable, due to the large size distribution (12–15% rms) of the core of the resulting capped nanocrystals. The large size distribution results in light emission over a wide spectral range. In addition, the reported preparation method does not allow control of the particle size obtained from the process and hence does not allow control of the color (i.e., emitted wavelength).
Danek et al. report the electronic and chemical passivation of CdSe nanocrystals with a ZnSe overlayer (Chem. Materials 8:173, 1996). Although it might be expected that such ZnSe-capped CdSe nanocrystals would exhibit as good or better quantum yield than the ZnS analogue, due to the improved unit cell matching with ZnSe, the resulting material remained only weakly luminescent (≦0.4% quantum yield).
Other references disclosing core-shell-type luminescent nanoparticles include Dabbousi et al. (1997) “(CdSe)ZnS Core/shell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites,” J. Phys. Chem. B 101:9463, Peng et al. (1997) “Epitaxial Growth of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability and Electronic Accessibility,” J. Am. Chem. Soc. 119:7019, and Peng et al. (1998) “Kinetics of II–VI and III–V Colloidal Semiconductor Nanocrystal Growth: Focusing of Size Distributions,” J. Am. Chem. Soc. 120:5343. Issued U.S. patents relating to core-shell nanoparticles include U.S. Pat. Nos. 6,207,229 and 6,322,901 to Bawendi et al. However, each of these references fails to provide any correction for structural mismatches in the lattice structures of the core and the shell.
Described herein is a method that provides, via the use of a reaction additive, a core-shell material displaying superior chemical, photochemical, and/or photophysical properties when compared to core-shell materials prepared by traditional methods. The method may produce shells that are better wed to the underlying cores. The method may also produce shells that are more electronically insulating to the core exciton. Additionally, this method may facilitate the controllable deposition of shell material onto the cores.